1. Field of the Invention
This invention relates to compositions and techniques useful in the measurement of ion concentrations in solution.
2. Description of the Background
Prior to the present invention, numerous methods for determining the concentration of metal ions in solution existed. These included atomic absorbtion photometry, ion-specific electrodes, flame photometry, and classical precipitation techniques.
Among the more recent techniques developed is a solid-phase colorimetric determination of potassium, which involves a nonpolar organic film, typically plasticized polyvinyl chloride, containing the ionophore valinomycin. The film is incubated with an aqueous solution containing potassium ions and a detectable anion, typically erythrosin B. The amount of erythrosin B retained by the film after washing can be measured by absorbance or reflectance and is directly related to the potassium concentration.
A second recently developed technique relies on the extraction of an ion pair into an organic solvent. This spectrophotometric method relies on the selective complexing of potassium by a specific macrocyclic polyether (crown ether), with the subsequent formation of an ion pair with a colored anion. The colored anion is extracted into an organic solvent, and the absorbance is measured to determine the amount of potassium ion present in the original solution.
A number of specific dye-coupled ionophores or dye-ionophore pairs have been prepared or described for use in such systems.
Although both of these new techniques eliminate a number of the disadvantages of prior techniques, such as requirements for a significant amount of wet chemistry or expensive equipment, they nonetheless suffer from several disadvantages. The solvent-extraction method requires the use of an immiscible liquid phase and a corresponding number of separation and clarification steps. The solid-phase method requires the existence of a photometer the reads either reflectance or absorbance in the solid-phase, such instruments being relatively rare in clinical laboratories.
Accordingly, there remains a need for a homogeneous assay capable of detecting the presence of metal ions in aqueous solutions that utilizes standard spectrophotometric equipment.